Apparatus and Process For Drilling A Borehole In A Subterranean Formation

ABSTRACT

An apparatus and process is disclosed for drilling a borehole into a subterranean formation with reverse circulation of drilling fluid. A tubular drill pipe extends into a subterranean formation. A bottom hole assembly connected to the drill pipe includes a drill bit for excavating the subterranean formation. A downhole motor is adapted for receiving electrical power from a cable extending into the subterranean formation. A downhole pump is powered by the motor and is configured for reverse circulating drilling fluid from the annular space surrounding the drill pipe to the interior space of the drill pipe. The drilling fluid is pumped upwards in the drill pipe by the downhole pump to carry excavated cuttings upwards through the interior space of the drill pipe.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and relates to an earlier filed U.S.provisional application Ser. No. 61/556,986 which was filed in theUnited States Patent and Trademark Office on Nov. 8, 2011.

FIELD OF THE INVENTION

The field of the invention is directed to apparatus and processes fordrilling a well by employing reverse circulation of drilling fluid.

BACKGROUND OF THE INVENTION

When conducting drilling to construct deep wells, the pressure ofdrilling fluid or drilling mud that is pumped down from the surface andinto the open hole of the formation may be quite high. It is usuallyadvisable to maintain a fluid/mud weight above the formation pressure toprevent gas “kicks” or influxes from the wellbore. Furthermore, thefriction pressure of pumping into a drill string may be quitesubstantial. Thus, pressure is required to be applied to cause thedrilling fluid and cuttings to flow through the drill string, out intothe open hole, and up the annulus at an adequate rate.

Too much pressure applied in this process can cause other problems. Thatis, such fluid pressure applied at the surface also is applied to theopen hole of the subterranean formation. High pressures applied to anopen hole of a formation may cause the formation to fracture, with asubsequent sudden loss of drilling fluid into the formation. Such asudden loss of drilling fluid into the formation may have severeconsequences. In some instances, there is a very narrow “window” ofpressure that may properly be applied in the drilling of a well withoutexerting too much or too little pressure. That is, excess appliedpressure can fracture the formation. On the other hand, inadequatepressure may not properly carry the drilled cuttings up the annulus tothe surface. Thus, a pressure “window” exists that engineers mustobserve in planning the pressure to exert while drilling a well.

Sometimes, lost circulation materials or pills are applied into drillingfluid, and such materials travel out of the bit and adhere to theformation to prevent such fluid loss into the formation. But, suchmaterials may damage the formation and reduce the ability of theformation subsequently to produce oil and gas into the wellbore duringproduction operations. Such damage to the formation is undesirable, andtherefore the use of such materials is not always advisable.

Operating within the mud weight “window” allows engineers to improvedrilling efficiency and set casing at the best possible depth. If casingis set too shallow, well construction cost increases and well depth islimited. Sometimes, this may cause the production rate to becompromised. In other instances, the target formation may not beaccessible. Techniques that widen or open the window to be employed aredesirable.

Conventional drilling employs rotary rock bits to compress the rock,causing the rock to fracture into cuttings. Pulsed power drilling,however, is a method of constructing a wellbore by applying voltage intothe rock of a formation, which causes the rock to fail in tension ratherthan compression. High voltage pulses employed in pulsed power drillingmay cause an electrical arc in the rock that causes the rock to break inan electro-crushing process. One illustration of such a drillingtechnique is described in U.S. Patent Publication No. US 2009/0050371 A1to Moeny et al. (See “Moeny”). In such applications, drilling fluidflows down the drill string and out through passages in the bit near theelectrodes and then up the outside of the drilling apparatus within theannulus to bring rock cuttings to the surface. (US 2009/0050371 A1,paragraph 0109).

A technique or apparatus that is capable of reducing the risk offormation damage and allowing the use of a reduced bottom hole pressurein the drilling of deep wells would be very desirable. A drillingtechnique that is capable of allow cuttings to be brought to the surfaceusing a reduced flow rate of flow of drilling fluid is highly desirable.

The invention is directed to improved drilling apparatus and processes.

SUMMARY OF THE INVENTION

An apparatus and process for drilling a borehole into a subterraneanformation with reverse circulation of drilling fluid is provided. Theapparatus employs a means to transfer a supply of electrical powerdownhole either from a cable running down the bottom hole assemblycomponents or the use of “wired drill pipe” with the capability toconduct electrical energy downhole with electrical conductorsincorporated into the drill pipe body. The apparatus comprises a tubulardrill pipe extending into the subterranean formation, the drill pipehaving an interior space and an annular space on the exterior of thedrill pipe. A bottom hole assembly is connected to the drill pipe, thebottom hole assembly comprising a bit to excavate the subterraneanformation to form cuttings. A downhole motor is provided, the downholemotor being adapted for receiving electrical power from either the cableextending into the subterranean formation or the use of wired drill pipesupplying the electrical power. A downhole pump is powered by the motor,the downhole pump being configured for reverse circulating drillingfluid and cuttings upwards through the interior space of the drill pipe.

In one embodiment of the invention, the apparatus comprises a mechanismfor removing excavated cuttings from the drilling fluid and thenrecirculating the drilling fluid downwards through the annular space onthe exterior of the drill pipe. The bit may comprise a rotary rock bit.In other applications, the apparatus may have one or more electrodesconfigured for applying a pulsed voltage to excavate the formation withapplied pulsed power. The downhole pump may be a positive displacementpump. In some applications, such as certain pulsed power bitapplications, the bit may not rotate.

In some embodiments of the invention, the cross sectional area of theinterior space of the drill pipe is less than the cross sectional areaof the annular space, thereby minimizing the drilling fluid flow ratethat is required to carry excavated cuttings upwards through theinterior space of the drill pipe.

A downhole generator may be provided, in one embodiment of theinvention, for applying pulsed power to the bit. The drilling fluid maycomprise an electrically insulating formulation having a low level ofelectrical conductivity. In some applications, the drilling fluidcomprises a carbon-based material.

A process is disclosed for drilling a borehole into a subterraneanformation with reverse circulation of drilling fluid. The processcomprises extending a tubular drill pipe into the subterraneanformation, the drill pipe having an interior space and an annular spaceon the exterior of the drill pipe, the drill pipe having a proximal endnear the top of the wellbore and a distal end with an attached bottomhole assembly. An electrical cable or wired drill pipe extends into thewell to supply power to downhole apparatus. The bottom hole assembly maycomprise a drilling bit. A pump and a motor are provided within theborehole, the pump being powered by the motor. The pump is in fluidcommunication with the interior of the drill pipe. It may be possible tocirculate drilling fluid from the annular space to the interior space ofthe drill pipe. Drilling fluid with cuttings may be pumped upwardsthrough the interior space of the drill pipe. In some embodiments ofinvention, excavated cuttings may be removed from the drilling fluidnear the top of the wellbore and re-circulated downward through theannular space. A control system may be employed to regulate the pulserepetition rate of the electrodes.

BRIEF DESCRIPTION OF THE FIGURES

Figures are provided to illustrate specific embodiments of theinvention, but the invention is not limited to only the embodimentsillustrated in the Figures, but may extend to other variations thatwould be appreciated by a person of skill in the art of drilling.

FIG. 1 illustrates a schematic of one embodiment of the invention thatemploys a pulsed power drilling bit;

FIG. 1A shows a perspective view of the pulsed power drilling bitemployed in the apparatus of FIG. 1;

FIG. 2 shows an alternate embodiment of the invention with a drillingapparatus that employs a rotary rock drill bit; and

FIG. 2A shows a more detailed view of the rotary rock drill bit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may employ pulsed power drilling apparatus orrotary rock drilling apparatus with reverse circulation drilling.Reverse circulation drilling refers to drilling wherein the drillingfluid is passed down the annulus to the outside of the drill string ordrill pipe, and then circulated upwards through the drill pipe towardsthe upper end of the wellbore.

As used herein, “drilling” is defined as excavating or otherwisebreaking and driving through a subterranean formation substrate. As usedherein, “bit” and “drill bit” are defined as the working portion or endof a tool for providing cutting, drilling, boring, or breaking action ona substrate, such as rock. As used herein, the term “pulsed power” isthat which results when electrical energy is stored (e.g., in acapacitor or inductor) and then released so that a pulse of current athigh peak power is produced.

Referring to FIG. 1, a drilling apparatus 18 is disclosed for entry intoa wellbore 19 of a subterranean formation 20. A tubular drill pipe 22 isprovided with an interior space 24 inside the pipe, and an annular space26 outside the drill pipe 22. A bottom hole assembly 28 is connected tothe drill pipe 22 and is located, during drilling, at the lower portionof the wellbore 19. A bit 30 is configured to contact and break the rockof subterranean formation 20. FIG. 1 shows a pulsed power bit 30, butother bits may be employed as further described herein. Drilling fluidis circulated in reverse flow direction, such that the fluid withcuttings flows along direction arrow 40, and then along direction arrow32. Cuttings are dislodged by the bit 30 and transferred by way ofdrilling fluid along arrow 32 to upwards in the wellbore 19. A cable 36is provided for providing a steady source of electricity to downholemotor 34, which drives downhole pump 38 to move the drilling fluid.

FIG. 1A shows a perspective view of the bit 30, which in the embodimentof FIG. 1A is a pulsed power bit 30. For drilling larger holes, aconical bit may be employed, especially if controlling the direction ofthe hole is a primary concern. Such a bit 30 may comprise one or moresets of electrodes for creating the electro-crushing arcs and mayoptionally comprise mechanical teeth to assist the electro-crushingprocess. One embodiment of the conical electro-crushing bit has a singleset of electrodes arranged coaxially on the bit, as shown in FIG. 2A. Inthis embodiment, conical bit 30 comprises a center electrode 48, thesurrounding electrode 44, the housing 42 and mechanical teeth 46 fordrilling the rock. Either or both electrodes may be compressible. Thesurrounding electrode may have mechanical cutting teeth 50 incorporatedinto the surface to smooth over the rough rock texture produced by theelectro-crushing process. In this particular embodiment of theinvention, the inner portion of the hole is drilled by theelectro-crushing portion (i.e., electrodes 48 and 44) of the bit 30, andthe outer portion of the hole is drilled by mechanical teeth 46. Thisresults in high drilling rates, because the mechanical teeth have gooddrilling efficiency at high velocity near the perimeter of the bit, butvery low efficiency at low velocity near the center of the bit. Thegeometrical arrangement of the center electrode 48 to the ground ringelectrode 44 is conical. It should be recognized that many types ofpulsed power bit configurations could be employed in the practice of theinvention, and the invention is not limited to only the configurationshown in FIG. 1A. U.S. Patent Publication No. US 2009/0050371 A1 toMoeny et al. (See “Moeny”) describes various embodiments and technicalspecifications that may be employed in the application of pulsed powerdrilling, and is incorporated herein by reference. Further, other pulsedpower drilling apparatus and techniques may be employed. Otherembodiments of the invention may employ rotary rock bits that do notemploy pulsed power, as further described herein in connection withFIGS. 2 and 2A.

FIG. 2 shows an alternate embodiment of the invention of drillingapparatus 60 that employs a rotary rock bit to break the rock to form aborehole by compression upon the rock within subterranean formation 77.In this embodiment, a tubular drill pipe 52 comprises an interior space72 and an annular space 70 on the exterior of the drill pipe 52. A powercable 54 extends into the wellbore 53 and supplied electrical power todownhole motor 56, which drives downhole pump 58, which transportsdrilling fluid. A bottom hole assembly 62 is positioned upon the end ofdrill pipe 52, and comprises a bit 64. In the embodiment of FIG. 2, thebit 64 is a rotary rock bit. A reverse circulation process is employedto circulate the drilling fluid along direction arrow 66 and then intothe interior space 72 of the drill pipe 52. Drilling fluid picks up rockcuttings generated by bit 64 and transports them along direction arrow68 and arrow 74 upwards in drill pipe 52 in a reverse circulation flowdirection.

FIG. 2A illustrates rotary rock bit 64, which in this particular exampleis a tricone style bit 64. The bit 64 has teeth 76 for contact with rockof the subterranean formation 77.

As described previously, in the first illustrated embodiment of thepresent invention, as shown in FIGS. 1 and 1A, a drill bit is providedupon which is disposed one or more sets of electrodes. In thisparticular embodiment, the electrodes are disposed so that a gap isformed between them and the electrodes are disposed on the drill bit sothat they are oriented along a face of the drill bit. Electrodes betweenwhich an electrical current passes through a mineral substrate (e.g.,rock) are not on opposite sides of the rock. Also, in this embodiment,it is not necessary that all electrodes touch the mineral substrate asthe current is being applied.

The electrodes of the embodiments shown in FIGS. 1-1A are disposed on abit and arranged such that electro-crushing arcs are created in therock. High voltage pulses are applied repetitively to the bit to createrepetitive electro-crushing excavation events. Electro-crushing drillingcan be accomplished, for example, with a flat-end cylindrical bit withone or more electrode sets. These electrodes can be arranged in acoaxial configuration, as one example.

The electrocrushing drilling process does not require rotation of thebit, but in some instances bit rotation may be desirable. Theelectro-crushing drilling process is capable of excavating the holebeyond the edges of the bit without the need of mechanical teeth. Inaddition, by arranging many electrode sets at the front of the bit andvarying the pulse repetition rate or pulse energy to different electrodesets.

The invention may be provided in other arrangements not specificallyshown or described in this specification but within the general spiritand scope of the invention.

What we claim is:
 1. An apparatus for drilling a borehole into asubterranean formation with reverse circulation of drilling fluid, theapparatus comprising: (a) a tubular drill pipe extending into thesubterranean formation, the drill pipe having an interior space and anannular space on the exterior of the drill pipe; (b) a bottom holeassembly connected to the drill pipe, the bottom hole assemblycomprising a bit to excavate the subterranean formation to formcuttings; (c) a downhole motor, the downhole motor being adapted forreceiving electrical power from a cable extending into the subterraneanformation; and (d) a downhole pump powered by the motor, the downholepump being configured for reverse circulating drilling fluid andcuttings upwards through the interior space of the drill pipe.
 2. Theapparatus of claim 1 further comprising a mechanism for removing theexcavated cuttings from the drilling fluid and recirculating thedrilling fluid downwards through the annular space on the exterior ofthe drill pipe.
 3. The apparatus of claim 1 wherein the bit comprises arotary rock bit.
 4. The apparatus of claim 1 wherein the bit comprisesone or more electrodes configured for applying a pulsed voltage toexcavate the formation with applied pulsed power.
 5. The apparatus ofclaim 1 wherein the downhole pump is a positive displacement pump. 6.The apparatus of claim 1 wherein the bit does not rotate.
 7. Theapparatus of claim 1 wherein the cross sectional area of the interiorspace of the drill pipe is less than the cross sectional area of theannular space, thereby minimizing the drilling fluid flow rate that isrequired to carry excavated cuttings upwards through the interior spaceof the drill pipe.
 8. The apparatus of claim 1 further comprising adownhole generator for applying pulsed power to the bit.
 9. Theapparatus of claim 1 wherein the drilling fluid comprises anelectrically insulating formulation having a low level of electricalconductivity.
 10. The apparatus of claim 9 wherein the drilling fluidcomprises a carbon-based material.
 11. A process for drilling a boreholeinto a subterranean formation with reverse circulation of drillingfluid, the process comprising the steps of: (a) extending a tubulardrill pipe into the subterranean formation, the drill pipe having aninterior space and an annular space on the exterior of the drill pipe,the drill pipe having a proximal end near the top of the wellbore and adistal end with an attached bottom hole assembly, the bottom holeassembly comprising a bit; (b) excavating the formation with the bit toform cuttings; (c) providing a pump and a motor in the borehole, thepump being powered by the motor, the pump being in fluid communicationwith the interior of the drill pipe; (d) circulating drilling fluid fromthe annular space to the interior space of the drill pipe; and (e)pumping drilling fluid with cuttings upwards through the interior spaceof the drill pipe.
 12. The process of claim 11 further comprising thestep of: (f) removing excavated cuttings from the drilling fluid nearthe top of the wellbore; and (g) recirculating the drilling fluiddownward through the annular space.
 13. The process of claim 11 whereinthe bit comprises one or more electrodes, further wherein the excavatingstep (b) comprises applying a pulsed voltage to one or more electrodesto excavate the subterranean formation.
 14. The process of claim 11wherein the pump is a positive displacement pump.
 15. The process ofclaim 11 wherein the bit is a rotary rock bit, the process comprisingthe additional step of rotating the rotary rock bit to excavate theformation.
 16. The process of claim 11 wherein the cross sectional areaof the interior space of the drill pipe is less than the cross sectionalarea of the annular space, thereby minimizing the drilling fluid flowrate required to carry excavated cuttings upwards through the interiorspace of the drill pipe.
 17. The process of claim 13 further comprisinga downhole generator for applying pulsed voltage to the bit.
 18. Theprocess of claim 13 wherein the drilling fluid comprises an electricallyinsulating formulation having a low level of electrical conductivity.19. The process of claim 18 wherein the drilling fluid comprises acarbon-based material.
 20. The process of claim 13 wherein a controlsystem is employed to regulate the pulse repetition rate of theelectrodes.